The present invention relates to a photoreceptor used for electrophotography and, more in particular, it relates to an electrophotoreceptor having an organic photoconductor layer.
The photoreceptor having an organic photoconductor layer (hereinafter simply referred to as an OPC photoreceptor) has a merit capable of easily controlling the spectral sensitivity region and available at a reduced cost, and accordingly it has been popularized rapidly for electrophotographic application use.
As such OPC photoreceptors, although there has been known a single layer type of using a polymeric charge transfer complex, a double-layered photoconductive structure in which a charge carrier generation layer and a charge carrier transport layer are laminated has often been used in view of the sensitivity characteristics and there have been proposed various combinations of an organic or inorganic charge carrier generation layer and an organic polymeric charge carrier transport layer. The organic charge carrier generation layer and organic charge carrier transport layer of this type have been formed generally by dispersing or dissolving an organic functional material such as a charge carrier generation agent or charge carrier transport agent, alone or together with a polymeric material such as a polycarbonate into a solvent and then coating it on the surface of a substrate by means of dipping method or spraying method. However, the method of using the solvent has a problem that the solvent has to be selected so that respective layers are not dissolved to each other and a homogenous coating film can not be obtained easily since it is difficult to maintain coating conditions, as well as it has a disadvantage, for example, that an enormous installation cost is required for avoiding explosion danger or injury to health caused by the solvent vapor.
On the contrary, since a vacuum vapor deposition method used for forming inorganic charge carrier generation layers does not require any solvent, there has been proposed a method of forming a charge carrier generation layer by vapor-depositing under vacuum an organic compound, for example, anthrathene, naphthoquinone, pyrene, perylene, phthalocyanine and cyanine pigments. Such a charge carrier generation layer can function sufficiently at a thickness of less than about several .mu.m. However, if it is disposed on the surface of a photoreceptor, it is easily abrased resulting in degradation of the property. Accordingly, a charge carrier generation layer is at first disposed on a substrate and subsequently, a charge transport layer of greater thickness is laminated thereover to protect the charge carrier generation layer from abrasion.
But, the charge carrier generation layer disposed on the substrate has a problem that charge injection tends to occur from the electroconductor layer on the substrate to photocarriers, i.e., hole-electron pairs, generated under irradiation of light, thereby offsetting charges on the surface of the photoreceptor and making it difficult for the carrier transport, and the increase of the potential at a light area and the change of potential at a dark area arise when using over again. For preventing such a disadvantageous phenomena, it has been known effective to apply an insulative subbing layer of about 0.1 to 5 .mu.m in thickness between the electroconductor of the substrate and the charge carrier generation layer.
The subbing layer in such an OPC photoreceptor is formed, for example, by applying a coating material obtained by dissolving a polymeric material such as polycarbonate into a solvent to the surface of the substrate by means of dipping method or spraying method in either case where the charge generation layer is formed by vacuum vapor deposition method or solution coating method. However, the method of using the solution-type coating material has a problem that a solvent has to be selected such that respective layers are not dissolved to each other, and that a homogenous coating film can not be obtained easily since it is difficult to maintain the coating conditions, as well as has a disadvantage that an enormous installation cost is required for avoiding problems of explosion danger or injury to health caused by solvent vapor.
Then, upon coating the subbing layer, if the surface of the substrate is too smooth, it results in uneven sensitivity characteristics since it is difficult to make the thickness of the coating layer uniform. On the other hand, if the surface of the substrate is too rough, since the coating thickness has to be increased, the efficiency is poor in the coating and drying steps and, in addition, it is not preferred in view of the sensitivity characteristics. Accordingly, there has been a disadvantage that the surface roughness of the substrate has to be adjusted at a high accuracy such that it is within an optimum range.
In addition, there is also a drawback that the subbing layer formed by coating has no sufficient close bondability with the charge carrier generation layer formed thereover by vacuum vapor deposition.
Further, there has been proposed a method of forming the organic charge carrier transport layer also by vacuum vapor deposition in the same manner as described above.
One of them is a proposal of forming an organic charge carrier transport layer by means of vacuum vapor deposition of a poly-p-phenylene sulfide film (Japanese Patent Application Laid-Open (KOKAI) No. 60-59353), but this film has a drawback that it is highly electrically insulative and the spectral transmittance in the visible ray region is poor. Furthermore, it is difficult for co-evaporation of the charge carrier transfer agent and it has not yet been put to practical use.
Furthermore, it has been proposed a method of vapor-depositing under vacuum respective starting materials such as polyimide and polyamide on a substrate, heat-polymerizing them to form a polyimide and polyamide film (Japanese Patent Application Laid-Open (KOKAI) No. 50-197730; Iijima, et al: "Vacuum", vol., 28, No. 5 (1985)). However, the layer can not be used as a charge carrier transport layer since it has high electric insulation and show poor its optical transmittance, and the charge carrier transporting agent is destructed if it can be co-evaporated under vacuum, upon polymerization of the film.
In view of the above, the present inventors have studied the formation of a vapor deposition film of a polycarbonate, such polycarbonate having been known as a vehicle for the formation of a subbing layer or a charge transport layer by the solution coating method.
However, if general-purpose polycarbonate is used for vacuum vapor deposition, only a film with a molecular weight of not greater than 2,000 can be obtained, which is insufficient for the close bondability with the substrate or the scratch resistance and can not be put to practical use. Further, if it is intended to vapor-deposit under vacuum a bishydroxy compound and a carbonic acid ester respectively as the starting material for the ester-exchange synthesis of polycarbonate, the homogenous vacuum vapor deposition is not possible due to excess difference in the vapor pressure and it has a drawback of contaminating the inside of a vacuum vapor deposition device making it impossible for practical use.
Then, as a result of the present inventors' further studies, it has been found that vacuum vapor deposition film of the polycarbonate can be formed by using an oligomer synthesized previously by an ester-exchanging process of a bishydroxy compound and a carbonic acid ester as a evaporation source for the polycarbonate. The present invention has been accomplished based on such a finding.